The present invention relates to a pneumatic radial tire. More particularly, the invention is concerned with a radial tire having a wandering preventing performance which is suitable for use on heavy vehicles, such as trucks and buses.
In recent years, radial tires have been used on heavy load vehicles, such as trucks and buses as well, because of excellent high-speed performance and wear resistance and low fuel consumption thereof. In the radial tires for use in such heavy-load vehicles, since the reinforcing belt layer of the tread portion is generally made of a metallic cord in order to withstand a heavy load applied thereto, the tread portion has a very high rigidity.
However, the high rigidity of the tread portion of the above-mentioned heavy-load radial tire is further increased as a result of the use of the radial tire in such a state that it is filled with a high-pressure air, which tends to cause a problem with respect to the occurrence of the so-called wandering phenomenon when the tire travels on a "rutted" road having recesses on its surface. That is, when the above-mentioned radial tire having a high rigidity is urged to pass over a rut during the travelling in the rut, an excessive lateral force is applied to the tire from the surface of the road, which leads to the failure in the control of a steering wheel, i.e., causes the wandering phenomenon.
Since camber thrust is a major causative factor of the wandering phenomenon, tires having a narrow groove provided in both the shoulder portions along the circumferential direction of the tire were proposed in order to prevent this phenomenon. In this expedient, the narrow groove is provided in order to decrease the shear rigidity of the shoulder portion relative to the lateral force, which relaxes the external force, thus contributing to the elimination of the failure in the control of a steering wheel.
However, the studies conducted by the present inventors revealed that the above-mentioned provision of a narrow groove in the shoulder portion tended to bring about the cracking at the bottom of the narrow groove along the circumferential direction of the tire when the narrow groove was located near the end portion of the shoulder. Further, it was found that, when the narrow groove was provided near the central portion of the tread for the purpose of preventing the occurrence of the above-mentioned cracking, the tread development width later to be described was substantially lowered, which led to another problem that the wear resistance was lowered.